The overall objective of the proposed research is to elucidate the role of Discoidin Domain Receptor 1 (DDR1) in the spread of breast tumor cells to the bone. Unlike other receptor tyrosine kinases, DDR1 is not activated by soluble growth factors but by fibrillar and basement membrane collagens. DDR1 is expressed during normal mammary gland development and overexpressed in primary ductal breast carcinoma. Knockout mice lacking DDR1 fail to develop functional lactogenic tissue. So far, we have identified 5 different isoforms of DDR1, which originate from alternative splicing in the juxtamembrane region. The hypotheses are: 1) DDR1 functions as a homing receptor in breast to bone metastasis. 2) Breast tumor cells aggressively colonize bone tissue, if the two full-length isoforms DDRla and DDRlb are not present. The specific aims to test these hypotheses are: 1) To re-express DDRla and -b in the highly metastatic human breast tumor cell line MDA-MB 231, which lacks endogenous expression of kinase-active DDR1. These cells will be either constitutively or inducibly transfected with full length DDR1 a and -b. The potential of DDRl-transfected cells to adhere to bone matrix and to degrade ECM in vitro will be compared to control cells. Because previous work showed that DDR1 triggers the release of proteolytic enzymes, including matrix metalloproteinase MMP3 and MMP10, we will particularly focus on the expression of these MMPs in order to further characterize pathways leading to destruction of basement membranes and bone tissue. 2) To study breast to bone metastasis in vivo by injecting MDA-MB 231 cells overexpressing full length DDR 1 into the left cardiac ventricle of nude mice. Compared to control animals, we expect to observe reduced formation of bone metastasis in animals with DDRl-overexpressing cells. By introducing GFP into MDA-MB 231 cells, we will monitor effects of DDR1 signaling during extravasation and metastatic progression in tissue sections or live animals. 3) To employ a set of microarray chips previously established in my laboratory, which are designed to specifically study the regulation of ECM-related genes in either human or mouse tissues. These arrays will be used to identify genes up- or down-regulated in bone tumor lesions from mice injected with MDA-MB 231 overexpressing full-length DDR1. We will be able to selectively profile the expression of ECM-related genes in the tumor using human arrays and host-specific responses using the mouse arrays. With the proposed research we want to evaluate the role of DDR1 as a so far unrecognized, but important therapeutic target in bone metastasis. A survey of the CRISP database showed that the NIH currently funds no other research on DDR1.